Airbag devices for protecting an occupant by inflating an airbag during a vehicle collision, and pretensioners for removing a slack of a webbing of a seat belt device by rapidly winding up a predetermined length of the webbing are well known. For activating such an airbag device or a pretensioner, it is required to determine the occurrence of a vehicle collision and the magnitude of the vehicle collision (crash severity). The crash severity correlates with the relative velocity to the object and the mass and the rigidity of the object.
Conventionally, the acceleration is detected for determining the magnitude of the collision. For example, in Japanese Patent Unexamined Publication No. 11-78769, the crash severity is determined on the bases of the magnitude of acceleration, the variation with time of the acceleration, and the variation with time of the velocity. In this publication, the variation with time of the velocity is obtained from the values by integrating the acceleration with time.
A collision determining method by detecting the deformation of an exterior member of a vehicle due to the collision is disclosed in Japanese Patent Unexamined Publication No. 11-78770. Distortion sensors are mounted on the exterior member of the vehicle so as to detect the deformation amount and the deformation speed of the exterior member, thereby determining the magnitude of the collision. It also should be noted that the protection of an occupant will be sometimes referred to as “restraint”. In addition, acceleration means acceleration (the derivative with respect to time of speed) acting on a vehicle in the decelerating direction during a collision.
A collision determining method by detecting the deformation speed of a front part of a vehicle during deformation of the vehicle body due to the collision is disclosed in Japanese Patent Unexamined Publication No. 2001-171476. According to this publication, acceleration sensors are disposed at two predetermined locations on a vehicle body so as to detect the compressive deformation speed of the vehicle body between the two sensors, thereby determining the magnitude of the collision.
In the collision determining method by detecting the acceleration disclosed in Japanese Patent Unexamined Publication No. 11-78769, the result of collision determination and crash severity is affected by the structure of a vehicle around a collided portion, the rigidity of the object, and the like.
In the collision determining method disclosed in Japanese Patent Unexamined Publication No. 11-78770, the deflection speed of the exterior member can be detected by the sensors mounted on the exterior member. However, the detected deflection speed is not always the deformation speed of the vehicle body structure in an impact loading direction. In addition, since the deformation stroke of the exterior member is generally short, it is difficult to detect the deformation speed on the way of progress of collision deformation.
Therefore, it is difficult to determine in an early stage of collision whether the collision is a collision between the vehicle and a small-mass high-velocity object with which only the exterior member is deformed (a collision not creating such a vehicle deceleration that occupant restraint by a passive safety system is needed) or a medium-velocity barrier collision with which the vehicle body is largely deformed (a collision creating such a vehicle deceleration that occupant restraint by a passive safety system is needed). That is, it is difficult to determine the occurrence of collision and the crash severity from the deformation speed of the exterior member.
The collision determining method disclosed in Japanese Patent Unexamined Publication No. 2001-171476 is not a method of detecting the deformation speed of the vehicle front end portion. As one of the acceleration sensors is located at the vehicle front end, an excess impact over the rated value of the sensor is exerted on the sensor in an early stage of a collision so that the mounting portion of the sensor is deformed, thus shifting off the detection axis of the sensor. With the shifted detection axis of the sensor, the deformation speed of the vehicle front end can not be properly detected.